Method for controlling the combustion air supply in a steam generator that is fueled with fossil fuels

ABSTRACT

A method for controlling the combustion air supply in a steam generator that is fueled with fossil fuels, the combustion air being supplied gradually in different combustion zones. The combustion air supply is controlled, depending on the NOx and/or CO content in the flue gas, in such a manner that first the air supply is varied between the different combustion zones with approximately constant air volumes. An external control of the overall air volume overrides this type of control.

The invention relates to a method for controlling the combustion air supply on a steam generator fired with fossil fuels in which the combustion air is added in stages in a plurality of combustion zones arranged one after another in the direction of the flue gas flow and in which the combustion air supply is also measured out as a function of the amount of fuel.

An air supply in stages of this type is known for example in the firing system of steam generators operated with pulverized lignite.

With the combustion of solid granular fuels, such as lignite, limit values must in any event be kept for the emission of nitrogen oxides and carbon monoxide. Finally, the firing system must be optimized in terms of efficiency; in other words, the fuel consumption and CO₂ emissions should be as low as possible.

It is known that when the boiler is operated overstoichiometrically, nitrogen oxides are produced; but when the amount of air is correspondingly reduced, carbon monoxide results. Both nitrogen oxides as well as carbon monoxide are undesirable in the flue gas.

For this reason for some time now air control for firing systems is being carried out with feed in stages, in other words in a first combustion zone in the direction of the flue gas flow somewhat less air is added in order to hinder the formation of nitrogen oxide. Carbon monoxide that is thereby produced is combusted later through the addition of combustion air in at least one downstream combustion zone.

In steam generators that are operated with pulverized lignite a plurality of pulverized fuel burners are mostly disposed above one another in the first combustion stage of the boiler, which constitutes the first combustion zone. The burnout of carbon monoxide is achieved through the addition of so-called overfire air charges in a first and second overfire stage above the burner assembly.

Downstream from the boiler the oxygen concentration in the flue gas as well as the carbon monoxide concentration in the flue gas are measured. The oxygen concentration in the flue gas is an indicator for the flue gas amount; the carbon monoxide content in the flue gas should not exceed certain limits.

Upon an increasing carbon monoxide concentration in the flue gas, the approach that was normally taken in the prior art was to increase the total amount of air supplied to the boiler. In the prior art this occurred uniformly at all air ports; in other words, the burner air and the overfire air charges were increased uniformly. As a result, the amount of flue gas that is emitted by the steam generator increases overall. This is not desirable, in particular because of the changes it causes in the convective heat transfer at the downstream heating surfaces. A change in the amount of flue gas always results in variations in efficiency.

In the control of the combustion air supply practiced in the prior art, each air feed into the boiler was controlled by means of an air curve stored in a controller for the corresponding air feed. The air curve showed the amount of air required relative to the firing rate of the boiler. The air curves were indeed related to each other in process engineering terms, however they generally were processed in isolation from each other. When the air curves were modified on the basis of changed burning conditions, all of the settings had to be recalculated and reinput. This is particularly complex with respect to control technology.

Therefore the object of the invention is to improve a method of the type referred to above with respect to control stability. In particular, the method of the invention should permit the steam generator to be operated with a flue gas quantity that is as constant as possible.

The object of the invention is achieved by a method of the type referred to above, which is characterized by the fact that control of the combustion air supply as a function of the NOx and/or CO concentration in the flue gas is carried out in such a way that at first a variation in the air supply between the various combustion zones is achieved with an approximately constant amount of air.

The invention may be summarized as follows: In the invention an internal control and an external control of NOx and/or CO is provided and these controls interact with each other so that, when there are NOx/CO fluctuations/deviations, at first the air supply between the individual combustion zones is varied while the amount of air remains largely constant.

If under these constraints a maximum permissible CO value cannot be kept, an external control system that adjusts the total amount of air used in the steam generator intervenes.

In this way a control that is especially insensitive to control fluctuations can be implemented.

Of course, the total air demand of the steam generator is also determined on the basis of the required fuel amounts and the calorific value of the fuels.

Preferably the air supply is varied by means of at least three combustion zones that are disposed in a series in the direction of flue gas flow.

In accordance with the invention, when an NOx limit value is exceeded, the air supply in a first combustion zone seen in the direction of flue gas flow is reduced, and the air supply in the last combustion zone seen in the direction of flue gas flow is increased correspondingly.

Assuming there are at least three combustion zones, this produces a kind of a cascade control.

If a CO limit value is exceeded, the air supply in the first combustion zone seen in the direction of flue gas flow is increased and the air supply in the last combustion zone seen in the direction of flue gas flow is decreased correspondingly.

When a specified amount of air in the last combustion zone is exceeded, the air supply in the next upstream combustion zone can be increased. Conversely, the air supply in the next upstream combustion zone may be decreased as needed.

In a preferred version of the method of the invention the amount of air that is supplied to each combustion zone is determined as a function of an air/fuel ratio (A number) that is specified for each combustion zone. The air/fuel ratio of each combustion zone may be specified, for example, as a function of the firing rate/load.

A version of the method in which the total air demand of the firing system is determined for the last combustion zone by means of the air/fuel ratio of this combustion zone using a fuel-specific air demand and the fuel mass flow is preferred.

In an especially preferred version of the method in accordance with the invention the oxygen concentration in the flue gas downstream from the firing system is calculated on the basis of the air/fuel ratio specified for the last combustion zone.

The oxygen concentration calculated in this manner may be used as a setpoint for an external control of the total amount of air.

This is particularly advantageous because in this way it is not necessary to store setpoint curves for the oxygen concentration of the flue gas over various firing system load states. The external control of the total amount of air is achieved by comparing a measured oxygen concentration downstream from the boiler with a calculated oxygen concentration that is obtained, as will be explained below, exclusively from the air/fuel ratio specified for the last combustion zone.

The fuel-specific air demand is advantageously determined by means of a continuous fuel analysis.

The invention also relates to a method for the control in a lignite-fired boiler of claim 1, wherein the fuel and at least one first partial stream of the combustion air is fed into burner stage as a first combustion stage of a combustion chamber and at least one additional partial stream of the combustion air is fed in as overfire air downstream in the direction of flue gas flow in at least one downstream overfire stage.

Each of the combustion stages including the burner stage forms a combustion zone and a variation of the air supply takes place across at least three combustion stages including the burner stage. The burner stage forms the first combustion stage of the boiler, and a plurality of pulverized fuel burners are disposed there mostly above one another. For the sake of simplicity this area of the boiler is referred to as a burner stage or a first combustion stage; however, in strict geometrical terms this is not a single stage, but rather the lower area of the combustion seam of the boiler.

The invention may be comprehended by a person skilled in the art without difficulty as meaning that the control scheme in accordance with the invention may be applied to any firing system using fossil fuels with air supply in stages and that the type of fuel being burned basically does not constitute a limitation upon the method. Thus, for example when steam generators are fired with anthracite, the use of air stages within the burner is known. The object of such a air supply in stages is also to minimize NOx emissions as well as CO emissions; here the goal is to improve the control stability of the air supply in stages with respect to steam generator efficiency fluctuations.

The invention is explained below on the basis of the attached drawings.

The drawings show:

FIG. 1: a graphical representation of the effects of the control scheme upon which the invention is based,

FIG. 2: a graphical representation of the air mass flow fed into the steam generator, and

FIG. 3: a schematic diagram of an air control system on a steam generator in accordance with the invention.

The method in accordance with the invention is explained below using as an example the control of the combustion air supply to a lignite-fired steam generator in which the lignite together with the primary air fed into pulverized fuel burners is injected into the boiler in the burner stage by means of pulverized fuel burners and is burned there. In addition, air is fed into the combustion process relative to the flue gas stream by means of the secondary air supply to the burners and by means of overfire air supplies fed in downstream relative to the flue gas stream.

Generally multiple burners are arranged in groups, mostly above one another, in the boiler of the steam generator. Combustion takes place in the immediate vicinity of the burner flames as well as above the burner flames that extend into the boiler. The combustion chamber of the boiler is divided into three combustion stages, whereby a first combustion stage is formed by the burner stage, a second combustion stage is defined by overfire air feed 1 (OFA 1), and a third combustion stage is defined by overfire air feed 2 (OFA 2).

In steam generators that are operated with pulverized lignite, the quality of the coal is subject to variations, which frequently are large. Many coals are rich in alkalis; and alkalis are known to be slag-forming substances.

Tests have shown that when certain coals are used, it is advantageous with regard to the fouling and slaging behavior in the combustion chamber and in the hot downstream heating surfaces to increase the air/fuel ratio above the burner stage. In the example embodiment described below, an air/fuel ratio of λ=1.05 above the burner stage is found to be favorable with regard to the fouling and slaging behavior and the resulting time between overhauls of the steam generator.

It is known that a slightly overstoichiometric operation of the burner (λ=1.05) is critical with regard to compliance with maximum NOx limit values. On the other hand, substoichiometric operation of the firing system is critical for compliance with CO limit values, and it also contributes to the formation of solid, baked-on deposits.

For this reason it is known, as already described above, that the amount of air required for combustion may be added in stages and a partial stream of the combustion air may be added with the primary and secondary air from the burners of the firing system, a further partial stream of the combustion air may be fed in as overfire air 1 in the overfire stage located above the burner stage, and an additional partial stream may be fed in downstream in the direction of flue gas flow as overfire air 2 in the third combustion stage.

In accordance with the invention, the total amount of the air fed into the steam generator is determined as a function of the fuel mass flow and the quality of fuel used and also as a function of the NOx and CO emissions from the steam generator. In an internal control system, which is explained below, the air supply is at first varied within the various combustion stages as a function of the measured NOx/CO emissions. The purpose of this control system is to keep the total amount of air supplied to the steam generator as constant as possible, within certain limits, at a particular firing rate.

In the control system an air/fuel ratio curve is stored for each combustion stage (see the upper diagram in FIG. 1), in other words the desired air/fuel ratio is specified as a function of firing rate. The air demand of the individual combustion stage is determined from the air/fuel ratio relative to the firing rate at which the unit is running. As suggested in FIG. 2, a total amount of air at the end of the combustion chamber is determined based on the air/fuel ratio in the last combustion stage (air/fuel ratio OFA 2) and a fuel-specific air amount as well as the fuel mass flow.

In accordance with the invention an NOx controller, which reduces the secondary air at the burners when a specified NOx setpoint is exceeded and adds the reduced amount of secondary to the overfire air 2, is provided downstream from the boiler. When a specified CO setpoint is exceeded, the overfire airflow rate in OFA 2 is reduced stepwise in the direction of the burner, if possible while maintaining the NOx setpoint, and the amount of air that is reduced with OFA 2 is added to the secondary air stream of the burner.

In the controller of the invention the required amount of air determined at any given combustion stage in each case is determined as the total amount of air subtracting the already added amount of air. The setpoint for the overfire air 1 is then determined from the stored air/fuel ratio for overfire air 1, and the amount of air that is already added up to this stage (essentially the burner air amount) is subtracted from the calculated value. The calculation for overfire air 2 proceeds in a similar manner. Here the overfire air 1 and the burner air amount are subtracted from the calculated total airflow.

The oxygen content is calculated from the air/fuel ratio specified for the last combustion stage using the formula O₂=21−21:λ. The calculated value is used as the setpoint for the external control of the total amount of air downstream from the boiler, and the oxygen content is used as an indicator of the total amount of flue gas emitted. As a result of the measure in accordance with the invention in which, using the formula stated above, a setpoint for the oxygen concentration of the flue gas is determined from the calculated air/fuel ratio for the last combustion stage, the storage of setpoint curves for the oxygen concentration of the flue gas as a function of the firing rate of the boiler becomes unnecessary. The respective value is represented as the function generator f(x) in the control scheme in FIG. 3; and this function generator is also shown in the formula 21−21:f(x). The terms in the control scheme shown in FIG. 3 mean: X=multiplication function, Σ=summation function, f(x)=function generator, and Pl=P1 controller (proportional/integrative). 

1. A method for controlling a combustion air supply of a steam generator fired with fossil fuels, the steam generator including a firing system having a plurality of combustion zones arranged one after another in a direction of flue gas flow the method comprising: controlling the combustion air supply as a function of the amount of fuel consumed in the combustion zones; directing portions of the combustion air supply to each of the combustion zones; and varying the combustion air supply portions directed to each of the combustion zones as a function of the NOx or CO concentration in the flue gas whereby the combustion air supply is substantially a constant amount of air.
 2. A method of claim 1, wherein said variation of the air supply portions is carried out across at least three different combustion zones.
 3. A method of claim 1 wherein if a NOx limit value is exceeded, the air supply portion in a first combustion zone seen in the direction of flue gas flow is decreased, and the air supply portion in a last combustion zone seen in the direction of flue gas flow is increased correspondingly.
 4. A method of claim 1 wherein if a CO limit value is exceeded, the air supply portion in a first combustion zone seen in the direction of flue gas flow is increased and the air supply portion in a last combustion zone seen in the direction of flue gas flow is decreased correspondingly.
 5. A method of claim 3, wherein if a specified amount of air in the last combustion zone is exceeded, the air supply portion in a next upstream combustion zone is increased.
 6. A method of claim 4, wherein the air supply portion in the next upstream combustion zone is decreased as needed.
 7. The method of claim 1 wherein the air supply portion supplied to each combustion zone is determined as a function of an air/fuel ratio (A number) to be specified for each combustion zone.
 8. A method of claim 7, wherein the air/fuel ratio of each combustion zone is specified as a function of the firing rate.
 9. A method of claim 7 wherein the air supply portion for a last combustion zone is determined by means of the air/fuel ratio of this combustion zone, by means of a fuel-specific air demand, and by means of the fuel mass flow.
 10. A method of claim 8, wherein the oxygen concentration in the flue gas downstream from the firing system is calculated on the basis of the air/fuel ratio specified for the last combustion zone.
 11. A method of claim 10, wherein the calculated oxygen concentration of the flue gas downstream from the firing system is used as a setpoint for an external control of the total amount of air.
 12. A method of claim 1 wherein a measured oxygen concentration of the flue gas is used as an actual value for the control of the total amount of air.
 13. A method claim 1 wherein the fuel and at least one first partial stream of the combustion air is fed into a burner stage as a first combustion stage of a combustion chamber and at least one additional partial stream of the combustion air is fed in as overfire air downstream in the direction of flue gas flow in at least one downstream overfire stage.
 14. A method of claim 13 wherein two additional partial streams of the combustion air are fed in as overfire air downstream in the direction of flue gas flow in two downstream overfire stages. 